The present invention relates in general to the telecommunications field and, in particular, to a method and apparatus for improving bit error performance within a hard decision radio interface processing environment.
Economic feasibility of wireless links between portable devices and their associated accessories require implementation of low-power, low-cost equipment. Many existing portable devices use infrared links to meet this requirement. Although infrared transceivers are inexpensive, they have limited range, are sensitive to direction, and can generally only be used between two devices. By contrast, radio transceivers have a much greater range, can propagate around objects and connect to many devices simultaneously.
Many radio transceivers use hard decision type radio receivers. A radio receiver is said to use hard decisions when the size of the channel alphabet (i.e., the number of possible signals that can be transmitted) equals the size of the demodulator output alphabet (i.e., the number of possible output signals from the radio demodulator).
For instance, the Bluetooth system uses a binary signal alphabet for which the channel symbol duration time is T=10xe2x88x926 seconds (sec). The output of a radio receiver implementation for that system might be discrete in amplitude, but continuous in time. Consequently, a switch between the two possible amplitudes can take place at any instant of time. As such, the Bluetooth receiver is a hard decision receiver.
In general, the baseband processing in any binary digital communication system requires a bit stream rate of 1/T bits/sec. Therefore, the output of the time continuous demodulator needs to be converted to a discrete time sequence. However, a significant design problem is to determine the correct sampling time or phase of the discrete time sequence. Baseband processing performance for each module in a Bluetooth system depends on how well a receiver can attain an accurate timing synchronization with the transmitter of the information sequence. An inferior timing synchronization will severely degrade the bit error rate (BER) performance of a receiver within a Bluetooth system. Additionally, to avoid the loss of information at the beginning of the information bit stream which will further degrade the BER performance of the receiver, an accurate timing synchronization needs to be attained within a relatively short period of time.
Referring to FIG. 1, an eye-diagram 50 of a received signal is plotted to illustrate the problem of choosing a correct sampling time or phase within a hard decision radio interface processing environment. The eye-diagram 50 represents a time before the hard decisions have been performed (in order to clarify the issue). As illustrated, all points above zero represent a xe2x80x9c+1xe2x80x9d, while all points below zero represent a xe2x80x9cxe2x88x921xe2x80x9d. The horizontal axis 51 represents the duration of one channel symbol and the four vertical bars 52-55 represent four possible sampling phases. Clearly, in this example, the best phase to choose is the second sampling phase 53 since it more or less is positioned in the middle of the eye opening. If the first sampling phase 52 or third sampling phase 54 is chosen, the result would be a decreased tolerance to noise and timing shifts between transmitter and receiver. Obviously, the fourth sampling phase 55 is a bad choice and should be avoided.
Referring to FIG. 2, each channel within a Bluetooth system is divided into 625 microsecond intervals or slots 30. One packet 34 of information can be transmitted per slot 30. Each packet 34 includes an access code 31, a header 32 and a payload 33. The access code 31 can be used for packet identification, synchronization and compensation for offsets.
FIG. 3 illustrates a radio 70 and correlator 74 which can be used for timing synchronization within a Bluetooth system. In this example, there are four possible phase values that should be checked, thus, the following discussion is in terms of establishing and comparing four phase samplings. Timing synchronization is accomplished by oversampling the demodulator output four times, i.e., f=1/Ts=4/T, where f is the sampling frequency and Ts is the sampling time. Subsequently, the oversampled binary sequence is correlated with a known access code which precedes the actual data. Generally, the known access code is also a binary sequence. For soft decision type receivers, the correct phase choice would be the phase that generates a maximum correlation with the known binary sequence. Unfortunately, hard decision type receivers make the task of finding the correct sampling phase more difficult than if a soft decision would have been available. Given hard values, there is potentially more than one phase that results in maximum correlation. Additional details of the sample selection process are described below.
The blocks 75 depicted in FIG. 3 represent memory elements in a shift register 72. For each phase sample, the contents of the shift register 72 is shifted one step to the right. Thus, the shift register 72 contains four sampling sequences of the received bit stream. An additional register 71 contains the known sequence with which the four sampling sequences are correlated. A correlation value or correlator output 76 is determined by the correlator 74 for each sample and compared with a threshold value by means of a threshold comparator 77. The threshold comparator 77 can be part of the correlator 74 or a separate process or device.
In a straight-forward implementation, the correlator output exceeds a predetermined threshold value, the timing is assumed to be correct and the phase of the sample values that caused the correlator trigger is used as a reference for the sampling time of the remaining part of the packet 34 (FIG. 2). Generally this approach works because the access code words are carefully chosen in a way that ensures good autocorrelation results. In other words, for all reasonable threshold values, the correlator output is unlikely to trigger unless the entire access code is within the correlator window. Thus, the phase that generates the first trigger output (binary representation of an indication that the correlator output 76 exceeds the preset threshold value) is chosen, unless a better value is found within a few sampling intervals. However, this type of phase decision within a hard decision radio interface processing environment is not optimal because the chosen phase value may be closer to the beginning of the eye rather than in the middle of the eye (FIG. 1). An erroneous choice of a sampling phase causes a receiver to be more sensitive to thermal noise and other interference from radio sources in the used frequency band. Consequently, an erroneous choice leads to degraded BER performance.
The Bluetooth system is only one example of a wireless system which uses a hard decision radio interface processing environment. The present invention relates to all digital radio interface processing environments which use hard decision decoding.
In accordance with a preferred embodiment of the present invention, a method and apparatus for improving bit error performance within a hard decision radio interface processing environment is provided, whereby a sampling unit samples an input signal to generate a sampling stream, and a correlator computes the correlation between the generated sample stream and a predetermined bit sequence. The correlation values are fed into a threshold comparator utilizing a variable threshold, and a phase decision unit determines the optimum sampling phase from the resultant threshold comparator values.
An important technical advantage of the present invention is that a method and apparatus for improving bit error performance within a hard decision radio interface processing environment is provided, whereby baseband processing of the receiver attains accurate timing synchronization with the transmission of the information sequence.
Another important technical advantage of the present invention is that a method and system for improving bit error performance within a hard decision radio interface processing environment is provided, whereby the required signal-to-noise ratio of the receiver is reduced.
Another important technical advantage of the present invention is that a method and apparatus is provided which adapts to a better channel by updating the comparator threshold value and thereby decreasing the probability of a false alarm.
Still another important technical advantage of the present invention is that the behavior of the receiver can be modified by changing the contents of a phase decision table.